1. Field of the Invention
The instant invention relates to a wide bandgap semiconductor device and a method for manufacturing the wide bandgap semiconductor device, which are particularly adapted for high voltage applications. The invention particularly relates to a field effect transistor (FET) using wide bandgap semiconductor as a base material and a method for manufacturing the FET.
2. Description of the Related Art
Historically, early stage in semiconductor industry, silicon (Si) material having a bandgap energy Eg=1.1 eV, or the gallium arsenide (GaAs) material having a bandgap energy Eg=1.4 eV has been firstly adopted for practical use. Compared with these preceding semiconductor materials, other semiconductor materials having wider bandgap energy Eg than these Si and GaAs are now referred as xe2x80x9cwide bandgap semiconductorxe2x80x9d. For example, zinc telluride (ZnTe) having a bandgap energy Eg=2.2 eV, cadmium sulfide (CdS) having a bandgap energy Eg=2.4 eV, zinc selenide (ZnSe) having a bandgap energy Eg=2.7 eV, a gallium nitride (GaN) having a bandgap energy Eg=3.4 eV, zinc sulphide (ZnS) having a bandgap energy Eg=3.7 eV and diamond having a bandgap energy Eg=5.5 eV are well known as the examples for the wide bandgap semiconductors. In addition, silicon carbide (SiC) is another example of wide bandgap semiconductor. The bandgap energy Eg of 2.23 eV is reported for 3Cxe2x80x94SiC, 2.93 eV is reported for 6Hxe2x80x94SiC, and 3.26 eV is reported for 4Hxe2x80x94SiC.
Generally, these wide bandgap semiconductors are superior in thermal and chemical stability, and wide bandgap semiconductors are superior in radiation immunity. In particular, because SiC is superior in these characteristics, applications in various industrial fields are expected. For example, SiC FETs are expected for high frequency devices and power devices having high reliability and stability.
In a channel region of an earlier SiC FET, a depletion layer is extended from in a gate region when a reverse bias is applied to a gate electrode so as to block current of carriers flowing from a source region to a drain region from, thereby achieving the off-state (See Japanese Patent Laid-Open No. 2000-299475). And the current flowing from the source to drain regions is controlled by drain voltage in an on-state by drain voltage. In other words, the negative feedback effect ascribable to the resistance in the channel caused by depletion layer at pinch off potential provides saturated drain current versus drain voltage characteristics, wherein the depletion layer extends to a drift region from the interface between a p-body regions and the drift region. In this way, switching operations of high current with high voltage is expected for the SiC FETs.
However, in the earlier SiC FETs, it is necessary to electrically insulate the gate electrode from the source region, which provides the limitation of the miniaturization of cell. And, there was a problem that reduction of the specific on-resistance normalized by chip area is not enough. In addition, there was another problem that the performance of the Schottky barrier formed between the gate electrode and SiC layer is determined uniquely by the kind of metallic material being employed in the Schottky gate electrode.
In addition, it is difficult to obtain a high breakdown voltage in the Schottky gate electrode structure. To obtain a high breakdown voltage, it is better to use nickel (Ni) among various metallic materials, because Ni has a high work function. However, because the etching of Ni film by photolithographic process is difficult, we must employ the lift-off process to delineate the Ni Schottky gate electrode, which unsuitable for the miniaturization of the SiC devices.
In view of these situations, it is an object of the present invention to provide a wide bandgap semiconductor device, which can achieve the miniaturization of the wide bandgap semiconductor device, reducing enough the specific on-resistance of the wide bandgap semiconductor device, which is normalized by the chip area of the wide bandgap semiconductor device.
Another object of the present invention is to provide a wide bandgap semiconductor device having high breakdown voltage and the manufacturing method of the wide bandgap semiconductor device, by which a Schottky gate structure having a desired barrier height can be formed selectively and easily.
To achieve the above-mentioned objects, a feature of the present invention inheres in a wide bandgap semiconductor device encompassing: (a) a drift layer of a first conductivity type made of a wide bandgap semiconductor material; (b) a body region of a second conductivity type opposite to the first conductivity type made of the wide bandgap semiconductor material, disposed at the top surface of and in the drift layer; (c) a source region of the first conductivity type made of the wide bandgap semiconductor material, disposed at the top surface of and in the body region; (d) a channel layer of the first conductivity type made of the wide bandgap semiconductor material, disposed at the top surface of and in the body region neighboring to the source region and further disposed at the top surface of and in the drift layer; and (e) a gate electrode including semiconductor layer at the bottom so that the semiconductor layer directly contact with the top surface of the channel layer, the semiconductor layer made of a semiconductor material having a different bandgap energy from that of the wide bandgap semiconductor material.
Another feature of the present invention inheres in a wide bandgap semiconductor device made of wide bandgap semiconductor material for controlling current flowing from source means to drain means by gate means, comprising: (a) drift means for transporting carriers by drift field between the source means and drain means; (b) body means disposed in the drift means for storing the carriers to be injected into the drift means; (c) source means disposed in the body means for providing the carriers so that the carriers can serve as the current flowing from the source means to the drain means; (d) channel means disposed at the top surface of the body means neighboring to the source means and further disposed at the top surface of the drift means for providing a current path between the source means and drain means; and (e) gate means directly contact with the top surface of the channel means so that an edge of gate means reaches to the source means, for controlling potential in the channel means, and simultaneously for achieving electrical isolation between the gate means and source means.
Still another feature of the present invention inheres in a method for manufacturing a wide bandgap semiconductor device, encompassing the steps of: (a) forming a drift layer of a first conductivity type made of a wide bandgap semiconductor material on a base material made of the wide bandgap semiconductor material; (b) forming a body region of a second conductivity type opposite to the first conductivity type made of the wide bandgap semiconductor material at the top surface of and in the drift layer; (c) forming a source region of the first conductivity type made of the wide bandgap semiconductor material at the top surface of and in the body region; (d) forming a channel layer of the first conductivity type made of the wide bandgap semiconductor material at the top surface of and in the body region neighboring to the source region and further at the top surface of and in the drift layer; (e) depositing a semiconductor layer having a different bandgap energy from that of the wide bandgap semiconductor material directly on the channel layer; and (f) doping impurity atoms from the top surface of the semiconductor layer.
Yet still another feature of the present invention inheres in a method for manufacturing a wide bandgap semiconductor device, encompassing the steps of: (a) forming a drift layer of a first conductivity type made of a wide bandgap semiconductor material on a base material made of the wide bandgap semiconductor material; (b) forming a body region of a second conductivity type opposite to the first conductivity type made of the wide bandgap semiconductor material at the top surface of and in the drift layer; (c) forming a channel layer of the first conductivity type made of the wide bandgap semiconductor material at the top surface of and in the body region neighboring to the source region and further at the top surface of and in the drift layer; (d) depositing a semiconductor layer having a different bandgap energy from that of the wide bandgap semiconductor material directly on the channel layer; (e) delineating the semiconductor layer so as to form a window portion, exposing a portion of the body region, and to form a pattern of a gate electrode; and (g) doping the first conductivity type impurity atoms from the top surface of the semiconductor layer so as not dope the bottom portion of the semiconductor layer, and simultaneously doping the portion of the body region exposed in the window portion, thereby simultaneously forming the gate electrode directly contacting with the top surface of the channel layer and a source region of the first conductivity type at the top surface of and in the body region.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the present invention in practice.